Four fire engines and around 25 firefighters attended a fire at a property on Northwood Way in Norwood.
Part of a split-level maisonette was damaged by fire. Three adults were taken to hospital by London Ambulance Service to be treated for smoke inhalation.
The fire is believed to have been caused by the unsafe disposal of smoking materials.
A London Fire Brigade spokesperson said: "If you're a smoker, it’s absolutely vital you ensure your cigarette is completely out when you’ve finished smoking it. If you don’t, you risk causing a fire which could not only destroy your home but also cost you your life.
“Firefighters recommend that you have a look at our online Home Fire Safety Checker to make sure there are no hidden hazards in your home.
“The online tool will ask you a series of questions that will help us work out the level of risk in your home, or the home of someone you care for. It only takes a few minutes to get tailored advice to keep yourself and loved ones safe from fire.”
The Brigade received the first of 12 calls about the fire at 0005 and Control Officers mobilised crews from West Norwood and Norbury fire stations to the scene. The fire was extinguished by 0123.